Scientists have detected a large underwater “plume” of oil from the Deepwater Horizon accident in the Gulf of Mexico last April, which spilled almost 5 million barrels of oil into the sea until the leak was successfully capped last month.

The discovery of a 650ft-high plume of hydrocarbon chemicals some 22 miles long by 1.2 miles wide, and 3,000ft below the surface of the Gulf, helps to answer the question of where the oil from the disaster has gone.

Two weeks ago, the influential US National Oceanic and Atmospheric Administration (NOAA) reported that only about 26 per cent of the oil from the spill remained in the environment. The rest had mostly evaporated or had dispersed, skimmed or burned off from from the sea surface, the NOAA scientists said.

Jane Lubchenko, head of the NOAA, said at the time that she did not believe there were any major quantities of oil still lurking as underwater slicks.

However, the latest study by the Woods Hole Institution in Massachusetts found a vast plume of hydrocarbons deep below the sea surface following a detailed survey of the area in June. They also found evidence to suggest that the underwater oil was not being rapidly degraded by marine microbes, as the NOAA scientists had predicted.

Richard Camilli, the chief scientist on the Woods Hole study published in the journal Science, said the findings were conclusive proof that an underwater plume exists and that it had been drifting slowly away from the location of the oil spill at the time the study was carried out.

The plume has shown that the oil is persisting for a longer period than some scientists would have expected, said Dr Camilli. “Many people speculated that subsurface oil droplets were being easily biodegraded. Well, we didn’t find that. We found it was still there. The plume is not pure oil. But there are oil compounds in there,” he added.

Water samples extracted from the depths where the oil is lying are clear and odourless, according to Christopher Reddy, a marine geochemist at Woods Hole who was part of the research team. “We don’t know how toxic it is and we don’t know how it formed, or why. But knowing the size, shape, depth, and heading of this plume will be vital for answering many of these questions,” he said.

Benjamin Van Mooy, a principal investigator of the research team, said the finding could have significant implications. “If the oxygen data from the plume layer are telling us it isn’t being rapidly consumed by microbes near the well the hydrocarbons could persist for some time. So it is possible that oil could be transported considerable distances from the well before being degraded,” he said.

Dr Simon Boxall of the University of Southampton’s National Oceanography Centre, who was not part of the research team, said the study suggests that 5.5 tonnes of oil per day were finding their way into the deep water column. “It is worth noting that [the Woods Hole researchers] put this input at about twice that expected from natural seeps, which input microscopic globules of oil into the deep ocean in the Gulf as a background,” he said.

The amount of oil involved is not large and accounts for only a small percentage of the total oil released, Dr Boxall added.

The NOAA investigation found that the efforts to contain the spill in the Gulf, such as the direct recovery of oil as it spewed from the wellhead and dispersal with chemical sprays, accounted for about a third of the oil that leaked from the well following the explosion and fire on 20 April that killed 11 oil workers.